1. Field of the Invention
The present invention relates to a focal point detection device used in a still camera, a video camera, a television camera or the like.
2. Description of Related Art
As a focal point detection device, for example, a device using a phase difference technique, as shown in FIG. 9, has been conventionally proposed. Such a device is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2-24614. In FIG. 9, (J) represents an in-focus state in which an image of an object P is formed just on a mechanical image plane 2 of a photograph lens 1. Light imaged on the mechanical image plane 2 further passes through a field lens 4p and an aperture stop 4q, and thereafter passes through a pair of secondary optical systems 4a and 4b which are spaced away from an optical axis z of the photograph lens 1 by the same distance but in opposite directions. The light is then imaged again just on an array of light receiving elements 5a and 5b. In other words, the field lens 4p is arranged so that an exit pupil of the photograph lens 1 and entrance pupils of the secondary optical systems 4a and 4b are in conjugate relations. The mechanical image plane 2, therefore, serves as an object plane of a re-imaging optical system composed of the field lens 4p and the secondary optical systems 4a and 4b, while the array of light receiving elements 5a and 5b serves as an image plane thereof.
As a result, in the in-focus state, the object P is imaged just on the mechanical image plane 2. The object P is then re-imaged just as a point image with no spread approximately at the center of each of the light receiving elements 5a and 5b.
In FIG. 9, (F) represents a so-called front-defocus state in which the photograph lens 1 is sent out forward, i.e., toward the object P, from a position at which the photograph lens 1 is positioned in the in-focus state. In the front-defocus state, the image of the object P is formed at a position shifted forward from the mechanical image plane 2 by a distance z.sub.f. This image, therefore, is blurred on the mechanical image plane 2. Also, on the light receiving elements 5a and 5b, the images of the object P are blurred and expand as compared to those formed in the in-focus state. A central point of the blurred images is closer to the optical axis z than that in the in-focus state.
In FIG. 9, (R) represents a so-called rear-defocus state in which the photograph lens 1 is sent out backward from the in-focus state position. In the rear-defocus state, the object P is imaged at a position shifted backward from the mechanical image plane 2 by a distance z.sub.r. This image, therefore, is blurred on the mechanical image plane 2. Also, on the light receiving elements 5a and 5b, the images of the object P are blurred and expand as compared to those formed in the in-focus state. The central point of the blurred images becomes more distant from the optical axis z than that in the in-focus state.
Each defocused amount z.sub.f or z.sub.r corresponds to a distance between a position at which the image of the object P is formed and the mechanical image plane 2. The defocused amount z.sub.f or z.sub.r can be obtained based on a distance between centers of light amount distributions respectively formed on the light receiving elements 5a and 5b. The photograph lens 1 is then controlled to be moved in the optical axis direction by a driving device so that the image of the object P is formed on the mechanical image plane 2.
Recently, zoom ratios of zoom lenses have become larger. Focal lengths on telephoto sides thereof have also increased more and more. In a zoom lens, if a length ratio of an object to an image of the object in an optical axis direction is a longitudinal magnification .alpha. and a height ratio of the object to the image in a direction perpendicular to the optical axis is a lateral magnification .beta., then .alpha.=.beta..sup.2. Moreover, an absolute value .vertline..beta..vertline. of the lateral magnification .beta. becomes larger as a focal length f of a photograph lens increases. Thus, the longitudinal magnification .alpha. also becomes larger as the focal length f of the photograph lens increases. In other words, assuming that the object is moved in the optical axis direction by the same amount, the defocused amount of the image becomes larger as the focal length of the lens increases.
When the defocused amount of the image is assumed to be the same, however, an object-field focal point detectable region, in which the movement of the object in the optical axis direction can be detected, becomes smaller as the focal length of the lens increases. This is apparent from FIG. 10.
When the object-field focal point detectable region is small, in order to locate a focal position, it is necessary to move a part of the photograph lens 1 or the whole photograph lens 1 forward and/or backward until the photograph lens 1 reaches a position at which the focal position can be detected. When the photograph lens is moved in a focusing direction, there is no problem. However, when the photograph lens is moved in a direction opposite to the focusing direction, the in-focus state cannot be reached, irrespective of how much the lens is moved. After it is judged that the in-focus state cannot be reached, when the movement direction of the photograph lens is reversed so that movement in the focusing direction occurs, the photograph lens enters the object-field focal point detectable region and an automatic focusing operation is made practicable. However, in this case, there are various problems of, for example, difficulty of high-speed focusing, a blur of the image due to forward and/or backward movement of the lens, and shake of an image screen in a case of moving picture such as, for example, a video image or a television image.
As the focal length of the photograph lens becomes longer, the object-field focal point detectable region becomes smaller. Various drawbacks are present in the aforementioned conventional focal point detection device.
In FIG. 11, (F) shows one of the front-defocus states in the aforementioned conventional focal point detection device. In this state, the defocused amount is the largest while the focal point is detectable. As is apparent from (F) of FIG. 11, EQU h/(L-z.sub.f0)=d/z.sub.f0
where L is a distance from the exit pupil 1a of the photograph lens to the mechanical image plane 2, z.sub.f0 is the largest defocused amount on the front-defocus side when the focal point is detectable (the object-field focal point detectable region on the front-defocus side), h is a maximum radius of a bundle of light beams, which is used in the focal point detection device, at the exit pupil 1a of the photograph lens, and d is a maximum height of the bundle of light beams, used in the focal point detection device, from the optical axis on a conjugate plane 6 of the imaging devices with respect to a re-imaging optical system.
Therefore, the expression: EQU z.sub.f0 =Ld/(h+d) (1)
is obtained.
Moreover, (R) of FIG. 11 shows one of the rear-defocus states. In this rear-defocus state, the focal point is detectable and the defocused amount is the largest. Here, EQU h/(L+z.sub.r0)=d/z.sub.r0
where z.sub.r0 is the largest defocused amount on the rear-defocus side when the focal point is detectable (the size of the object-field focal point detectable region on the rear-defocus side).
Therefore, the expression EQU z.sub.r0 =Ld/(h-d) (2)
is obtained.
The radius of the bundle of light beams, which enters the focal point detection device, at the exit pupil is generally set to be constant irrespective of an imaging state thereof. The radius h of the exit pupil of the photograph lens, in other words, is constant both in the state shown by (F) having the largest defocused amount on the front-defocus side and in the state shown by (R) having the largest defocused amount on the rear-defocus side. Moreover, the maximum height d of the bundle of light beams from the optical axis, which can be detected by the focal point detection device, at the position 6 which is conjugate to the photograph device with respect to the re-imaging optical system, is determined by the limit of the focal point detection device. Therefore, a value of d is also constant both in the state shown by (F) having the largest defocused amount on the front-defocus side and the state shown by (R) having the largest defocused amount on the rear-defocus side.
In the conventional focal point detection device mentioned above, the mechanical image plane 2 of the photograph lens is arranged at the conjugate position 6 of the imaging devices with respect to the re-imaging optical system. Therefore, the height d corresponds to the maximum height on the mechanical image plane 2.
From the expressions (1) and (2), EQU z.sub.f0 &lt;z.sub.r0.
Thus, the size z.sub.f0 of the object-field focal point detectable region on the front-defocus side is smaller than the size z.sub.r0 of the object-field focal point detectable region on the rear-defocus side.
On the other hand, as described above, the focal length on the telephoto side becomes longer. This is because there is a great demand for telephotography. There are supposed to be many cases, therefore, in which the object P is positioned on the infinity side of the position in the in-focus state before the focusing operation. There are supposed to be more front-defocus states than rear-defocus states.
However, in the aforementioned conventional focal point detection device, the size z.sub.r0 of the object-field focal point detectable region on the rear-defocus side is larger than that on the front-defocus side. Accordingly, the conventional focal point detection device does conform to actual circumstances in which the front-defocus state occurs more frequently than the rear-defocus state.
In addition, if an existing focal point detection device can be applied to lenses having different image screen sizes, then cost can be reduced. However, when a conventional focal point detection device for a large image screen size is used with a lens having a small image screen size, the size of a region in which a measurement is conducted is too large. This results in an undesirable increase in an error in selection of a photographic subject.